Camptothecin (Chem. Abstracts Registry No. 7689-03-4) is a naturally occuring compound found in Camptotheca acuminata (Nyssaceae) which has antileukemic and antitumor properties. Numerous camptothecin analogs having like properties are known, examples being those described in U.S. Pat. No. 4,894,456 to Wall et al. and European Patent Application No. 0 325 247 of Yaegashi et al.
A number of syntheses for camptothecin are known. Several routes are reviewed in Natural Products Chemistry, Vol. 2, 358-361 (K. Nakanishi, T. Goto, S. Ito, S. Natori and S. Nozoe eds.) and in J. Cai and C. Hutchinson, Camptothecin, in The Alkaloids, Vol. XXI, 101-137 (Academic Press 1983). The biosynthesis of camptothecin is described in Natural Products Chemistry, Vol. 3, 573-574 (K. Nakanishi et al. eds.). A recent synthetic route is described in U.S. Pat. No. 4,894,456 to Wall et al. (see also references cited therein).
A problem with prior methods of synthesizing camptothecin is that they are largely linear syntheses. Such syntheses provide low yields of the final product because of the sequential loss in product during each step of the total synthesis. Parallel syntheses (i.e., a strategy in which two synthetic paths are followed separately and the products thereof combined to form the final product) provide higher yields, but few such syntheses have been available for camptothecin. Accordingly, an object of the present invention is to provide a parallel synthetic method for making camptothecin and analogs thereof.